1. Field of the Invention
The present invention relates generally to air supply control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a plurality of turbosuperchargers accompanying with the internal combustion engine.
2. Description of the Prior Art
In an internal combustion engine equipped with a turbosupercharger, a superior response in supercharging would not be expected, especially, under a situation wherein the engine is operating at a relatively low speed in the case where the turbosupercharger is arranged to be large in supercharging capacity and further it would not be expected that a combustion chamber in the engine is supercharged sufficiently as occasion demands in the case where the turbosupercharger is arranged to have an improved response in supercharging.
In view of the above, there has been proposed to provide a couple of turbosuperchargers of primary and secondary to an internal combustion engine and to make only the primary turbosupercharger operate when intake air mass flow in an intake passage of the engine is relatively small and make both the primary and secondary turbosuperchargers operate simultaneously when the intake air mass flow is relatively large, as disclosed in, for example, Japanese patent applications published before examination under publication numbers 56-41417 and 59-160022, respectively, and Japanese utility model application published before examination under publication number 60-178329. In such a case, a superior response in supercharging can be obtained even on the occasion of engine operation at a relatively low speed and a combustion chamber in the engine can be supercharged sufficiently on the occasion of engine operation at a relatively high speed with an arrangement in which the primary turbosupercharger is made to have an improved response in supercharging.
In connection with the internal combustion engine provided, as mentioned above, with the primary and secondary turbosuperchargers which are caused to work selectively so as to vary their supercharging capacity in accordance with operating conditions of the engine, first and second operating areas provided on an operating characteristic chart of the engine, which may be shown with an axis of abscissa representing engine speed and an axis of ordinate representing opening degree of throttle in the engine, are predetermined respectively for a first supercharging operation whereby only the primary turbosupercharger is to work and a second supercharging operation whereby both the primary and secondary turbosuperchargers are to work simultaneously. (Hereinafter, the first and second operating areas will be referred to as P and S operating areas, respectively.) These P and S operating areas are generally determined with reference to a condition in which the engine operates with an appropriate temperature after having been warmed up sufficiently.
However, in the engine which is provided with the primary and secondary turbosuperchargers and for which the P and S operating areas are fixedly predetermined as mentioned above, the primary and secondary turbosuperchargers are also caused often to work simultaneously when the engine is operating with a relatively low temperature before having been warmed up sufficiently and this results in a disadvantage that an exhaust gas which is discharged from the engine to be applied to an exhaust emission purifying catalyzer is made too low in temperature so as to be unfavorable for early activation of the exhaust emission purifying catalyzer. The reason why the exhaust gas is made too low in temperature is that the exhaust gas is used to drive both of the primary and secondary turbosuperchargers in a relatively low temperature environment and therefore loses a great deal of thermal energy at the primary and secondary turbosuperchargers so as to decrease the temperature thereof steeply and to be hard to increase in temperature when the primary and secondary turbosuperchargers work simultaneously.
In the engine which is provided with the primary and secondary turbosuperchargers and for which the P and S operating areas are fixedly predetermined, there is another disadvantage that the primary and secondary turbosuperchargers can not satisfy enough all desires in operation raised respectively in accordance with speed ranges taken selectively in a transmission which is connected to the engine for transmitting torque generated by the engine to a drive mechanism for driving wheels of a vehicle equipped with the engine.
In more detail, it is desirable that the combustion chamber in the engine is supercharged with quick response when a low speed range, such as the first or second speed range, is selected in the transmission for acceleration or deceleration, and further supercharged smoothly when a high speed range, such as the third or fourth speed range, is selected in the transmission for causing the vehicle to travel steadily at relatively high speed. Accordingly, the primary and secondary turbosuperchargers are desired to work simultaneously when the low speed range is selected in the transmission until the engine speed on the increase becomes relatively high and also when the high speed range is selected in the transmission until the engine speed on the decrease becomes relatively low. However, in the case where the P and S operating areas are fixedly predetermined, the primary and secondary turbosuperchargers can not comply appropriately with such a desire.
In the engine which is provided with the primary and secondary turbosuperchargers and for which the P and S operating areas are fixedly predetermined, there is a further disadvantage that the primary and secondary turbosuperchargers can not satisfy enough all desires in operation raised respectively in accordance with octane numbers of fuel supplied to the combustion chamber in the engine.
In more detail, when only the primary turbosupercharger is caused to work with a favorable response in supercharging, resistance against exhaust gas in an exhaust passage of the engine is increased so that a relatively large exhaust pressure is produced and therefore the engine is apt to be subjected to knocking. On the other hand, when both the primary and secondary turbosuperchargers are caused to work simultaneously, the resistance against exhaust gas in the exhaust passage is reduced and therefore the engine is advantaged in respect of knocking.
The knocking occurring on the engine is directly affected by the octane number of fuel supplied to the combustion chamber of the engine, and accordingly each of the primary and secondary turbosuperchargers is desired to work selectively in P and S operating areas which are determined variably in accordance with the octane number of fuel supplied to the combustion chamber of the engine. However, in the case where the P and S operating areas are fixedly predetermined, the primary and secondary turbosuperchargers can not satisfy the desire thereto.
Further, the engine which is provided with the primary and secondary turbosuperchargers and for which the P and S operating areas are fixedly predetermined, is apt to cause torque shock therein when the secondary turbosupercharger starts working in a situation where the primary turbosupercharger has worked, and the torque shock thus caused is ready to be large especially in the case where the engine is accelerated.
In more detail, when the engine is accelerated in an operating state of the P operating area in which only the primary turbosupercharger has worked, an accelerator is actuated suddenly on a large scale so that the operating state of the engine moves rapidly into the S operating area in which the primary and secondary turbosuperchargers are to work simultaneously. However, since a certain acting time is necessitated to cause the secondary turbosupercharger to start operating, such a situation that only the primary turbosupercharger works though the operating state of the engine has moved into the S operating area is temporarily brought about and therefore torque produced by the engine is reduced to be considerably small due to increased exhaust pressure in an exhaust passage extending from the engine. After that, the secondary turbosupercharger is started working to cause the engine to produce increased torque so that the torque produced by the engine is varied steeply and this results in the large torque shock caused when the engine is accelerated.